Coaxial hybrid structure employing ridged waveguide for reducing resonant modes



3 1968 J. D. CAPPUCCLI 3,364,

COAXIAL HYBRID STRUCTURE EMPLOYING RIDGED WAVEGUIDE FOR REDUCING RESONANT MODES Filed Aug. 25, 1964 2 Sheets-Sheet 1 FIG? I N VEN TOR,

Jan. 16, 1968 J. D. CAPPUCCI 3,364,444 COAXIAL HYBRID STRUCTURE EMPLOYING RIDGED WAVEGUIDE FOR REDUCING HESONANT MODES Filed Aug. 25, 1964 I 2 Sheets-Sheet 2 2 I30 5 |2b //l F '6. 7A FIG. 7B

22 32 K 4 V I I20 12b j 24 I //7' W Bu I36 |3b I 35 2 3 PR H EXCITATION E --EXCITATION I NVENTOR. JOSEPH D. CAPPUCCI ATTORNEYS United States Patent COAXIAL HYBRID STRUCTURE EMPLOYING RIDGED WAVEGUIDE FOR REDUCING RES- ONANT MODES Joseph D. Cappucci, Fairview, N.J., assignor to Merrimac Research and Development, Inc., Irvington, N.J., a corporation of New York Filed Aug. 25, 1964, Ser. No. 391,842 13 Claims. (Cl. 33311) ABSTRACT OF THE DISCLOSURE A coaxial hybrid structure utilizing ridge line waveguide type elements arranged to prevent propagation of undesired resonance modes. A strip line feed is utilized for the H arm excitation and a stub and multi-section transformer feed for E arm excitation to broaden the operating bandwith of the structure.

This invention relates to hybrid junction circuits and more particularly to hybrid circuits for use with coaxial transmission lines.

The theory and operation of hybrid circuits, often called hybrid tee circuits, are well known in the art of microwave systems for use with both waveguide and coaxial line transmission media. The present invention relates to hybrid structures for use with coaxial transmission lines although certain aspects of waveguide technology are utilized.

Hybrid circuits for use with coaxial transmission lines are usually four terminal devices. These devices operate in a manner such that energy applied to one of two input terminals splits substantially equally to be present at two of the other terminals, which serve as the output terminals, in either the same or opposite phase, and is isolated from the other input terminal. In one type of co axial hybrid tee circuit, which is described in United States Patent 2,935,702, issued May 3, 1960, to Bernard M. Dwork, and assigned to the same assignee, a four terminal structure is utilized having two input terminals coupled to two collinear output terminals. Energy applied to the first input terminal is split equally between the two collinear output terminals at the same phase and is isolated from the second input terminal. Energy applied to the second input terminal is split between the two collinear output terminals and appears at these terminals in opposite phase. This energy is reflected back from the first input terminal so that it does not appear thereon.

In the hybrid circuits disclosed in the Dwork patent, the structure is such that a problem arises due to the production of resonance modes. These resonance modes occur due to the presence of half wavelength resonant paths in the structures between various points, such as between two ground planes, which may be excited when the appropriate frequency energy is applied to either of the two input terminals. Of course, such resonance modes are desirably avoided. In the Dwork hybrid circuits an attempt is made to eliminate these resonance modes by the use of strip lines, used as chokes, inserted at selected places in the hybrid structures. While this alleviates the problem to a considerable extent, the configurations of the Dwork structures are such that all of the resonant paths cannot be eliminated, even with the chokes, and therefore some undesired resonance modes are still produced.

The present invention is directed to coaxial hybrid structures of the same general type as disclosed in the Dwork patent. Here, however, the problem of undesired resonance modes is substantially eliminated. In accordance with the invention hybrid coaxial circuits are pro- 3,364,444 Patented Jan. 16, 1968 "ice vided in which ridge line waveguide type transmission coupling is utilized throughout the hybrid structures. These ridge lines serve as chokes to prevent the propagation of undesired resonance modes. The ridge lines are also dimensioned, with respect to the input and output terminals of the structures, and the structures are so formed so that substantially no half wavelength paths capable of giving rise to the undesired modes can exist.

It is therefore an object of the present invention to provide coaxial hybrid structures utilizing ridge line coupling members.

A further object is to provide coaxial hybrid tee structures in which the excitation of unwanted resonance modes is substantially prevented by the use of ridge line coupling members which serve as chokes to prevent the formation of these modes.

Still another object is to provide a coaxial tee hybrid structure utilizing a ridge waveguide for coupling the input terminals to the collinear arm output terminals.

Other objects and advantages of the present invention will become more apparent upon reference to the following specification and annexed drawings in which:

FIGURE 1 is a schematic representation of a hybrid structure made in accordance with the invention;

FIGURE 2 is a longitudinal plan view, taken in section showing the details of a hybrid structure according to the invention;

FIGURES 3 and 4 are cross-sections of the hybrid structure of FIGURE 2 taken along lines 3-3 and 4-4, looking in the direction of the arrows; and

FIGURES 5, 6, 7A and 7B are cross sections of the hybrid structure of FIGURES 2-4 showing the electric field patterns at various places.

FIGURE 1 illustrates the general principles of a coaxial hybrid structure by means of a current flow analogy. A closed metallic housing 10 is provided With four coaxial type terminals 1, 2, 3 and 4. Only the respective center conductors 21, 22, 23 and 24 of each coaxial terminal is shown and the outer conductor portion of each one (not shown) is connected directly to the housing. A separate metallic ridge line or guide 12 and 13 is located on opposite Walls of the housing. Therefore, the housing 10 approximates a waveguide of the so-called double ridge type. The ridge lines 12 and 13 have openings therein to permit passage of the center conductors to the area between the two ridges, as shown.

Terminal I (sometimes called the H arm) is one of the input terminals and its center conductor 21 extends into the space between the two ridge lines where it is connected directly to the center conductors 22 and 23 of terminals 2 and 3 at a junction 15. Terminals 2 and 3 are the collinear output terminals. The center conductor 24 of terminal 4, which is the second input terminal (sometimes called the E arm), passes through the upper ridge line 12 and is connected to the lower ridge line 13.

Thus, the center conductor of terminal 4 is effectively a shunt probe in what may be considered to be a waveguide type cavity.

In operation, energy (current) i applied to terminal 1 splits into two substantially equal parts i and i at the junction 15 of the center conductors of terminals 1, 2 and 3. The phases of the currents appearing at the terminals is the same and no energy is further propagated in the cavity to be present at terminal 4. Current i ap plied at terminal 4 excites a field in the cavity which produces currents i and i at the junction 15 in the center conductors 22 and 23 of terminals 2 and 3. The

. currents i and i are opposite in phase. Any energy function of producing equal amplitude energy output, of the same or opposite phases, at the two collinear terminals while isolating the tWo input terminals from each other.

The function of the ridge lines or guides 12 and 13 is more fully described by referring to FIGURES 24 which show a preferred embodiment of a coaxial hybrid tee circuit constructed in accordance with the present invention. Similar reference numerals are used as in FIGURE 1, where applicable.

The housing for the hybrid is formed by several interconnected pieces of metal including side walls 70 and 71; top and bottom walls 72 and 73, on which ridge line structures 12 and 13 are located respectively; and end walls 74 and '75. The walls are made of a material having good conductive properties and they are held together by any suitable fastening means such as pins or screws (not shown). Illustratively, the hybrid housing is shown having a generally rectangular shape although any other suitable shape may be utilized, e.g., square.

The ridge lines or ridge guides 12 and 13 are illustratively of generally rectangular shape and are formed integrally as part of or held on the opposite top and bottom walls 72 and 73 of the hybrid housing. As shown, the ridge line 12 is on the same wall 72 that holds terminals 3 and 4 while ridge line 13 is on the same wall 73 that holds terminal 3. Terminal 1 is located on the end wall 74 and all of the terminals are held to their respective walls by any suitable fastening means (not shown) as is conventional.

The ridge lines are selected to have a predetermined height and width, as are the walls 70-73 of the hybrid housing defining the dimensions of the cavity to produce a predetermined cutoff frequency within the housing below which energy will not propagate. The selection of these dimensions is done in a manner determined in accordance with microwave waveguide theory, as is well known in the art.

Ridge line 12 has openings 42 and 44 formed therein to permit the passage of the central conductor 22 of terminal 2 to junction 15 and central conductor 24 of terminal 4 to the ridge line 13 respectively. Similarly, ridge line 13 has an opening 43 formed therein for permitting the passage of the central conductor 23 of terminal 3 to junction 15. For convenience ridge line 12 is designated as having three sections which are: section 12a from the end wall 74 of the housing adjacent terminal 1 to the opening 42; section 12b from opening 42 to opening 44; and section 120 from opening 44 to the other end wall 75. Similarly, ridge line 13 is designated for convenience as having a section 13a from the end wall 74 adjacent terminal 1 to opening 43; section 13b from opening 43 to the point at which central conductor 24 is connected thereto; and section 136 from the point of connection of conductor 24 to the other end wall 75.

As in FIGURE 1, the inner conductor 21 of terminal 1 is connected at junction point 15 to the inner conductors 22 and 23 of the two terminals 2 and 3. As seen most clearly in FIGURE 3, the center conductor 21 is preferably a strip conductor which is located approximately half-way between the opposing faces of the ridge line sections 12a and 13a. The center conductor 24 of terminal 4 passes through opening 44 in the ridge line 12 and is electrically connected to the opposite face of ridge line 13. The outer conductors 31, 32, 33 and 34 of terminals 1, 2, 3 and 4 are respectively connected to the outside of the hybrid housing. These outer terminals usually have some type of fitment, such as a screw thread or bayonet member for holding the coaxial line to be attached to the various terminals of the hybrid. These fitments, as well as the various insulating members and other hardware for the terminals have been omitted for the sake of clarity.

As can be seen in FIGURES 24, the outer conductors 32 and 33 of terminals 2 and 3 are connected directly to the respective ridge lines 12 and 13. The outer conductor 32 of terminal 2 is connected to the outer conductor 34 of terminal 4 through the ridge line section 1211. Similarly, the outer conductor 33 of terminal 3 is connected to the inner conductor 24 of terminal 4 through the ridge line section 13b. Also, the outer terminals 32 and 33 are connected to the outer terminal 31 of terminal 1 through the ridge line sections 12a and 130. respectively.

The operation of the resonator of FIGURES 24 is similar to that previously described with respect to FIG URE 1. Energy applied to input terminal 1 propagates down the strip line section 21 in a TEM mode between the inner faces of the ridge lines section 12a and 13a and the strip center conductor 21. The electric field pattern, as shown in FIGURE 5, is substantially identical from the center strip conductor 21 to the opposing face of each of the ridge lines due to the symmetry of the structure about center conductor 21. Because of both the symmetry and the mode of propagation, the energy introduced at input terminal 1 splits equally and is in phase at the junction 15 between the two collinear arm output terminals 2 and 3. None of the energy can propagate beyond junction 15 to terminal 4 since the ridge guide made of sections 1211 and 13b cannot propagate the TEM mode.

Energy incident at terminal 4 (E arm) of the structure is propagated through the coaxial line formed by its respective inner and outer conductors 24 and 44. The center, inner conductor 24 bridges the space between ridge lines 12 and 13 and terminates on the opposite ridge in a direct connection thereto. Thus, the center conductor 24 becomes a probe in the cavity. The outer conductor 44 is connected to ridge line 12.

As can be seen, the excitation applied from the coaxial line structure of terminal 4 is unbalanced. Since the ridge waveguide formed by housing 10 and ridges 12 and 13 is a balanced structure, a suitable coupling device is preferably provided to couple the coaxial line to the waveguide structure. This is accomplished by a balun (balanced-to-unbalanced transformation device) formed by a shunt stub of approximately one-quarter wavelength at the operating frequency. The stub for the balun comprises the conductors 12c and 13c of the ridge lines and the connecting portion 10 of the housing therebetween which includes end wall 75.

Energy incident at input terminal 4 propagates between ridge guide sections 12b and 13!) toward junction 15 in a TE mode. The coaxial line formed by the inner and outer conductors 24 and 44 and the probe formed by inner conductor 24 extending between the ridge lines, together with the balun ridge line section 120, 13c and 10, form a coaxial line to ridge guide transition.

The transverse electric field pattern of the TE mode energy propagating between ridge line sections 12b and 13b is shown in FIGURE 6. This is a balanced excitation and as such, it is antisymmetric about a center line (or plane) 60 transverse to the vertical dimension of the structure. The center line (or plane) 60 is an equipotential surface at ground potential over the length of ridge line sections 12b and 13b between probe 24 and junction 15. The TE anode energy propagates to the junction 15 which is symmetrically located between the inner opposing faces of the ridge line sections 12b and 13b, i.e., on the line or plane 60. Due to the antisymmetric properties of the TE mode about the line 60, energy couples out of the collinear terminals 2 and 3 in equal magnitude but in opposite phase.

The difference in the two excitations to terminals 1 and 4 is shown in FIGURES 7A and 7B respectively. As shown in FIGURE 7A, excitation of the H arm terminal 1 propagates in the TEM mode symmetrically from the center strip line 21 (see FIGURE 5 also) to the opposing inner faces of ridge line sections 12a and 13a. It then splits at junction 15 to the two collinear arms 2 and 3. In FIGURE 7B energy applied to E arm terminal 4 propagates in the TE mode between ridge line sections 12b and 13b (see FIGURE 6 also) to the junction 15 where it splits to the two collinear terminals in opposite phase. The TE mode energy also propagates down the TEM line path formed by the ridge line sections 12a and 12b and the center strip line 21. This excitation, which is unwanted at terminal 1, is short circuited by the end wall 74 of the housing, to which is connected outer conductor 31 of input terminal 1, so that no energy from input termnial 4 can propagate out of input terminal 1. The junction 15 is displaced a quarter wavelength from the short circuit plane of the end wall 74 of the housing 10 forming a shunt resonant stub. This provides a balun for matching the TE mode energy from the balanced ridge guide sections 12b and 13b to the unbalanced coaxial outputs of the collinear output terminals 2 and 3.

The use of ridge guides in cavity resonators or waveguides is well understood in the microwave art. As is known, ridge guides are effectively choking structures which lower the cutoff frequency of a resonator or waveguide from that which normally could be obtained due to the physical dimensioins of the waveguide or resonator itself. Also, as is known in the art, the use of a double ridge, one on each opposing face, lowers the cutoff frequency still further.

In the present invention, the ridge guide sections 12b and 13b connecting the outer terminals 32 and 33 of collinear terminals 2 and 3 to the outer end inner conductors 24 and 34 of terminal 4 is a choke structure which keeps the input energy excitation at terminal 1 from propagating to terminal 4. Similarly the other ridge guide sections 12a and 13a and 12c and 130 prevent the formation of undesired resonance modes.

Each of the ridge lines 12 and 13 is also formed with a number of steps 51, 52 and 53 to vary the height of the lines for selected distances. As shown, the steps on each of the lines 12 and 13 are symmetrical with each other and extend for the same length along the lines. The purpose of the steps 51, 52 and 53 is to vary the impedance characteristics of the ridge line. This provides a convenient way to obtain impedance matching within the hybrid.

Due to the use of the ridge guides 12 and 13 there is substantially no chance afforded for undesired resonance :modes to propagate within the hybrid. As can be seen in FIGURES 24 no half-wavelength path exists between any two terminals which is not connected by a choke (ridge :guide) Coaxial hybrid structures have been constructed in accordance with the present invention that have operated in the 4-8 kmc. range substantially free from all undesired resonance mode while at the same time providing efficient hybrid coupling. Of course, the principles of the present invention may be extended to other frequency ranges of operation.

While a preferred embodiment of the invention has been described above, it will be understood that it is illustrative only, and the invention is limited solely by the appended claims.

What is claimed is:

1. A hybrid tee for coaxial lines comprising:

first and second coaxial collinear transmission lines,

having their inner conductors connected,

a third coaxial line having a strip line inner conductor electrically connected to the inner conductors of said first and second coaxial collinear lines,

a fourth coaxial line,

a first ridge line means electrically connecting the inner conductor of said fourth coaxial line to the outer conductor of one of said first and second collinear coaxial lines,

and a second ridge line means electrically connecting the outer conductor of said fourth coaxial line to the outer conductor of the other of said first and second collinear coaxial lines, and means forming an impedance matching transformer having an even number of sections between the inner conductor of said fourth coaxial line and the junction of the inner conductors of said first, second and third coaxial lines, at least one of said first and second ridge line means having an impedance matching means which is effective between the inner conductor of said fourth coaxial line and the junction of the inner conductors of said first, second and third coaxial lines.

2. A hybrid tee as set forth in claim 1 and further comprising a first transformer electrically coupled between said fourth coaxial line and said first and second ridge line means to match the unbalanced input of the fourth coaxial line to the electrically balanced arrangement of said first and second ridge line means.

3. A hybrid tee as set forth in claim 2 wherein said first transformer includes at least one section of one of said ridge line means.

4. A hybrid tee as set forth in claim 2 and further comprising a second transformer electrically coupled between said first and second collinear coaxial lines and said first and second ridge line means to match the unbalanced outputs of said first and second collinear coaxial lines to the electrically balanced arrangement of said first and second ridge line means.

5. A hybrid tee as set forth in claim 4 wherein said second transformer includes at least a portion of one of said ridge line means.

6. A hybrid tee as set forth in claim 1 wherein said impedance matching means comprises steps on at least one of said ridge line means.

7. A hybrid tee as set forth in claim 6 wherein the steps form an even number of sections for said impedance matching means.

8. A hybrid tee for coaxial lines comprising:

a housing forming a cavity therein,

a respective first and second ridge line means with substantially fiat outer faces located on two opposing walls of said housing within said cavity and defining a space therebetween substantially throughout the length of the cavity,

first and second collinear coaxial lines having their inner conductors connected at a point within said cavity,

a third coaxial line having a strip line inner conductor located in the space between the opposing faces of said first and second ridge line means and which is connected to the inner conductors of said first and second lines,

and a fourth coaxial line having its inner conductor electrically connected to one of said ridge line means and its outer conductor connected to the other of said ridge line means, said one ridge line means being electrically coupled to the outer conductor of one of said first and second collinear coaxial lines and said other ridge line means being electrically coupled to the outer conductor of the other of said first and second coaxial lines, at least one of said first and second ridge line means having steps thereon forming an impedance matching transformer means having an even number of sections between the inner conductor of said fourth coaxial line and the junction of the inner conductors of said first, second and third coaxial lines.

9. A hybrid tee as set forth in claim 8 wherein the junction of the inner conductors of said first, second and third coaxial lines is spaced substantially one-quarter wavelength from the input of the first coaxial line into the cavity.

10. A hybrid tee as set forth in claim 8 further com prising a first balun transformer formed by said first and second ridge line means and said housing to match the unbalanced electrical input of said fourth coaxial line to the electrically balanced first and second ridge line means.

.11. A hybrid tee as set forth in claim 10 further comprising a second balun transformer formed by said first '7 and second ridge line means and said housing to match the unbalanced electrical outputs of said first and second coaxial lines to the electrically balanced first and second ridge line means.

12. A hybrid tee for coaxial lines comprising:

a housing forming a cavity therein,

first and second ridge line means on two opposing walls of said housing within said cavity thereby forming a choking structure,

first and second collinear coaxial lines having their inner conductors connected,

a third coaxial line having an inner strip line conductor located between the opposing faces of said first and second ridge line means and connected to the inner conductors of said first and second lines at a junction,

a fourth coaxial line having its inner conductor extending across the space between the opposing faces of said first and second ridge line means and electrically connected to one of said ridge line means and its outer conductor connected to the other of said ridge line means, said one ridge line means being electrically coupled to the outer conductor of one of said first and second collinear coaxial lines and said other ridge line means being electrically coupled to the outer conductor of the other of said first and second coaxial lines,

first balun transformer means including portions of said first and second ridge line means remote from said junction electrically coupled between said fourth coaxial line and said first and second ridge line means to match the unbalanced electrical input of said fourth coaxial line to the electrically balanced arrangement of said first and second ridge line means,

second balun transformer means including portions of said first and second ridge line means between said first coaxial line inputs to the cavity and said junction electrically coupled between said first and second collinear coaxial lines and said first and second ridge line means to match the unbalanced electrical outputs of said first and second coaxial lines to the electrically balanced arrangement of said first and second ridge line means and a dimension transition on at least one of said first and second ridge line means between the inner conductor of said fourth coaxial line and the junction of the inner conductors of said first, second and third lines to form an impedance matching transformer between said third and fourth coaxial lines. 13. A hybrid tee as set forth in claim 12 wherein said dimension transition forms an even number of sections for said impedance matching transformer.

References Cited UNITED STATES PATENTS 2,935,702 5/1960 Dwork 333-41 3,087,127 4/1963 Broghetti 33334 3,192,489 6/1965 Walker et al 333-11 FOREIGN PATENTS 972,318 6/1959 Germany.

HERMAN KARL SAALBACH, Primary Examiner.

ELI LIEBERMAN, Examiner.

R. COHN, S. CHATMON, In, Assistant Examiners. 

